security: address Copilot review on PR #36, add concurrency + compat tests

Copilot findings (PR #36):
- parsers: use trim_start() for blank/comment detection, parse
  untrimmed line to preserve GECOS whitespace
- pam: newline after password prompt is best-effort (let _ =)

New tests (#37):
- test_concurrent_lock_operations: 4 threads racing lock/unlock
- test_gnu_compat_status_output: compare -S output with GNU passwd
- test_gnu_compat_lock_unlock: verify lock/unlock matches GNU behavior

147 tests, zero clippy warnings.

fuzz/corpus/fuzz_login_defs_parse/01299a45f3ef4c5742cf587639698cefef137d24

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+
…

fuzz/corpus/fuzz_login_defs_parse/059c18facb6732dcdc9adc505034776ded23d192

@@ -0,0 +1,2 @@
+滩
+滩

fuzz/corpus/fuzz_login_defs_parse/069950a4d4766de004680f2d6eda34e40258cf9a

@@ -0,0 +1,2 @@
+
+W

fuzz/corpus/fuzz_login_defs_parse/07c342be6e560e7f43842e2e21b774e61d85f047

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+l

fuzz/corpus/fuzz_login_defs_parse/0842fc6997743ca83c80d593692f4d09bf370a4c

@@ -0,0 +1 @@
+ƻƻ

fuzz/corpus/fuzz_login_defs_parse/088fb1a4ab057f4fcf7d487006499060c7fe5773

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+